Conway's Game of Life
The first popular cellular automata based artificial life simulation. Life was invented by British mathematician John Horton Conway in 1970 and was first introduced publicly in "Scientific American" later that year.
Conway first devised what he called "The Game of Life" and "ran" it using plates placed on floor tiles in his house. Because of he ran out of floor space and kept stepping on the plates, he later moved to doing it on paper or on a checkerboard and then moved to running Life as a computer program on a PDP-7. That first implementation of Life as a computer program was written by M. J. T. Guy and S. R. Bourne (the author of Unix's Bourne shell).
Life uses a rectangular grid of binary (live or dead) cells each of which is updated at each step according to the previous state of its eight neighbours as follows: a live cell with less than two, or more than three, live neighbours dies. A dead cell with exactly three neighbours becomes alive. Other cells do not change.
Many hackers pass through a stage of fascination with Life, and hackers at various places contributed heavily to the mathematical analysis of this game (most notably Bill Gosper at MIT, who even implemented Life in TECO!; see Gosperism). When a hacker mentions "life", he is more likely to mean this game than the magazine, the breakfast cereal, the 1950s-era board game or the human state of existence.
["Scientific American" 223, October 1970, p120-123, 224; February 1971 p121-117, Martin Gardner].
["The Garden in The Machine: the Emerging Science of Artificial Life", Claus Emmeche, 1994].
["Winning Ways, For Your Mathematical Plays", Elwyn R. Berlekamp, John Horton Conway and Richard K. Guy, 1982].